1. Field of the Invention
The invention relates to internal combustion engines of the fuel-injection type equipped with a catalytic exhaust converter preceded by a sensor and, more particularly in such engines, a device and a process for diagnosis of the condition of the sensor disposed upstream from the catalytic converter.
2. Discussion of the Background
It is known how to use systems for modifying the quantity of fuel injected into an engine as a function of the exhaust-gas composition and, more particularly, of the oxygen content of these gases. To this end, the oxygen content is measured by means of a nonlinear sensor known as the "lambda" sensor or EGO sensor, where EGO is an English-language acronym for "Exhaust Gas Oxygen". Such a sensor is disposed upstream from the catalytic exhaust converter, and the signal delivered by this sensor is used to modify the quantity of fuel injected into the engine cylinders via a first feedback loop. For this reason, the sensor is also known as a richness-regulating sensor.
It is clear that poor condition of this sensor leads to poor operation of the engine and of the catalytic converter, in turn leading to pollutant emissions at abnormally high levels. It is therefore important to determine the condition of this sensor at all times in order to diagnose poor operation thereof when its condition has deteriorated beyond certain limits. The present solutions for diagnosis of the condition of the upstream sensor comprise analyzing the behavior of the sensor in response to richness excitations in open loop or closed loop and monitoring the following parameters:
the minimum voltage delivered by the sensor: if too high, a fault is indicated; PA0 the maximum voltage delivered by the sensor: if too low, a fault is indicated; PA0 the lean-to-rich transition time; if too long, a fault is indicated; PA0 the rich-to-lean transition time; if too long, a fault is indicated; PA0 the period of the signal delivered by the sensor in closed loop: if too long, a fault is indicated. PA0 perfect knowledge of the mechanisms of aging of the sensors, PA0 numerous tests to establish a relationship between the measured degradations of parameters and their effects on pollutant emissions. PA0 a filter circuit to which there is applied the second correction signal KRICH in order to deliver a filtered signal KRICH.sub.F, PA0 a measuring circuit to which there is applied the output signal V.sub.upstream of the upstream sensor in order to determine the mean value T.sub.m of the period of correction of the first control loop, and PA0 a logic circuit to determine, as a function of the values of the filtered signal KRICH.sub.F and of the mean period T.sub.m, whether the condition DIAG of the upstream sensor is good or defective. PA0 filtering of the second correction signal KRICH to obtain a filtered signal KRICH.sub.F, PA0 calculation of the mean value T.sub.m of the period of the output signal V.sub.upstream of the upstream sensor, PA0 comparison of the said filtered signal KRICH.sub.F with two values, the maximum KRICH.sub.max and the minimum KRICH.sub.min, to determine whether the condition DIAG of the said upstream sensor is correct or defective, according to whether the filtered signal KRICH.sub.F is respectively within the limits defined by the maximum and minimum values or outside the said limits for the value of the mean period T.sub.m.
The diagnosis then comprises declaring failure of the sensor if one or more faults are detected.
Such a diagnostic process is based on analysis of the sensor behavior in order co deduce therefrom a sensor condition on the basis of assumed degradation mechanisms. For example, as a sensor ages, its dynamic voltage range is reduced and/or its transition times become longer The disadvantage of such a diagnostic process is that a perfect correlation does not exist between these measurements and the emissions of pollutants.
In addition, calibration of fault detection thresholds proves to be very tricky and necessitates:
In addition, it is not possible in all cases to guarantee that the diagnosis is reliable. For example, a sensor with reduced dynamic voltage range may prove to be good with regard to pollutant emission if only that characteristic is affected.